Which description best explains the flow of $Q$ (heat)?

• A. $Q$ flows from a region of lower $T$ (temperature) to a region of higher $T$ without any external work.
• B. $Q$ only flows between objects that are in direct physical contact.
• C. $Q$ flows spontaneously from a region of higher $T$ to a region of lower $T$ until thermal equilibrium is reached.
• D. $Q$ can flow from cold to hot objects without any energy input.

Answer: (C)

Verified step by step guidance

1. Understand that heat transfer, denoted as \(Q\), is the energy transferred due to a temperature difference between two regions or objects.
2. Recall the second law of thermodynamics, which states that heat flows spontaneously from a region of higher temperature (\(T\)) to a region of lower temperature until thermal equilibrium is reached.
3. Recognize that this spontaneous flow does not require any external work or energy input; it is a natural process driven by the temperature gradient.
4. Note that heat transfer can occur through different mechanisms: conduction (requiring physical contact), convection, and radiation (which does not require physical contact). Therefore, \(Q\) is not limited to flowing only between objects in direct contact.
5. Conclude that the correct description of heat flow is: \(Q\) flows spontaneously from a region of higher temperature to a region of lower temperature until thermal equilibrium is reached.